In order to solve the problems linked with this hydrate formation risk, operators have to use water-base and/or oil-base well fluids containing additives such as hydrate formation inhibitors. The most commonly used additives are thermodynamic inhibitors whose main purpose is to challenge the temperature and/or pressure limits from which hydrates form. The most commonly used inhibitors are salts and glycols, which poses serious corrosion and toxicity problems while involving high-cost formulations. For oil-base fluids, it is possible to use emulsifying systems efficient in preventing hydrate crystal agglomeration in case of formation (dispersing surfactants or anti-agglomeration agents).
An interesting alternative to thermodynamic inhibition is the use of kinetic inhibitors that do not prevent hydrate formation and whose purpose is to delay the appearance of crystals or to slow down the crystal growth rate.
In order to anticipate hydrate formation risks, one can currently only base oneself on tests carried out with reactors or test loops on more or less simplified fluid formulations, or on model hydrates (THF or freon) allowing to work at atmospheric pressure. The main difficulty for testing formation kinetics in aqueous solutions is linked with the uncertain nature of the nucleation phenomenon that leads to non-reproducible measurements.
In order to overcome this difficulty, hydrates are formed from melting ice or hydrates are made to form by heterogeneous nucleation by adding solid particles to the solutions. At the present time there is no simple, fast and reliable method that is directly applicable, in the laboratory or on site, to real drilling fluids for temperatures close to 0° C. and under natural gas pressure.
The object of the present invention is to provide, in the laboratory or on a drilling site (mud logging cab), or on a production site, a reliable method of determining or analyzing the hydrate formation kinetics on a real well fluid by measuring the heat released upon hydrate crystallization at a given gas pressure, using the DSC (Differential Scanning Calorimetry) method. These measures will allow the operator to compare the kinetics observed with various mud formulations, containing kinetic inhibitors or not, and thus to select the mud formulation that is best suited to the drilling conditions in the case of drilling operations.
This method is in fact applicable to any type of aqueous solution (drilling fluid, cementing fluid, production fluid, production effluent) whose properties as regards gas hydrate formation kinetics have to be known in a reliable and reproducible way.
Gas hydrate formation is a crystallization process that requires a nucleation stage followed by a crystal growth stage. The nucleation process corresponds to the formation of nuclei or “critical germs” in the solution. These critical germs serve as growth sites for the future crystals. Generally, three nucleation types are considered (Mullin J. W., in: Crystallization, London, Butteworths, 1972): —homogeneous primary nucleation, which is spontaneous, —heterogeneous primary nucleation, caused by foreign particles, —and secondary nucleation induced by the crystals already formed.
Nucleation can take place only if the liquid sample breaks through a potential barrier from a thermodynamic point of view. The liquid therefore has to be cooled down below the liquid/solid equilibrium temperature, which means undercooling. Furthermore, formation of the germ being the result of local density fluctuations, the kinetic aspects must be taken into account. It appears that the lower the temperature, the higher the nucleation rate and, consequently, it is extremely difficult to observe crystallization in the neighbourhood of the equilibrium temperature. Another obvious consequence is that there is not one temperature at which crystallization is observed and that this temperature is statistically distributed. It is in fact possible to define only a more probable crystallization temperature, in the statistical sense of the term, which generally depends on the volume of the sample (Clausse D., in: Encyclopedia of Emulsion Technology, Becher P., ed., New York, Marcel Dekker, 1985, vol. 2, p. 77).
In the literature on gas hydrate formation kinetics, most authors emphasize the great experimental difficulty encountered to obtain reproducible results when they study the nucleation of hydrates from an aqueous solution. Many of them decide to overcome this problem by studying nucleation from melting ice. The stage that follows nucleation is crystal growth. The main parameters that govern this stage are the gas diffusion rate, the interfacial area, the pressure, the temperature and, of course, the undercooling degree. Several models have been developed to predict the gas hydrate formation kinetics, but there still is no satisfactory model, mainly because of the nucleation phenomena that are difficult to control experimentally and of the growth models that closely depend on the experimental set-up used (Sloan E. D., “Gas hydrate tutorial”, Am. Chem. Soc., Div. Fuel Chem., 42, 2, 449-456).
Considering all these experimental difficulties relative to hydrate formation kinetics, the present invention aims to use the high-pressure DSC (Differential Scanning Calorimetry) technique on stable emulsified systems in order to study the kinetics of gas hydrate formation in drilling fluids.